Hellomyfello's explanation was lacking on some parts, so i am going to redo the explanation.
Imagine you have a box which insides is completely isolated from the outsides, meaning that by no means can you know what happens inside when the box is closed. Such a box does not exist irl.
You then put a cat in it (can be any animal really), with it you have a radioactive source which has a 50% probability of having an nucleus releasing radiation after a certain ammount of time has passed. Just next to the radioactive source you have a geiger muller counter, which is an instrument that reacts to radioactivity. And if the radioactive source emits radiation, the counter will react, and it will have a hammer smash a glass containing cyanide, which will kill the cat.
And here comes the problem, according the the most popular interpertation of quantum mechanichs, the copenhagen interpertation, the state of any system is only decided when an outside observation is made. So because the cat is in a box which insides is completely sealed. Any people on the outside cant possibly know whenever the cat is dead or alive, meaning it is in a superstate between dead and alive. therefore you can only know whenever the cat is alive when you open the box, and the state of the cat only becomes decided when the box is opened and people look inside of it.
The thought experiment was created to show how absurd the copenhagen interpretation of quantum mechanics is. There is other interpretations that does not fall into the same trap as the copenhagen interpretation does, one of them is the many worlds interpretation, that states that both states of the cat is realized, meaning that the universe will split up, into one where the cat is alive and one where the cat is dead. But we cant possibly know from observation which of the many interpretations that is true.

I like to imagine it in like the saying "Life is like a box of chocolates, you never know what you are going to get." I like to use Schrodinger's cat in a similar fashion. "All possible outcomes exist simultaneously, you won't find the true answer until you actually act, ie open the box."

Schrödinger's cat: a cat, a flask of poison, and a radioactive source are placed in a sealed box. If an internal monitor detects radioactivity (i.e. a single atom decaying), the flask is shattered, releasing the poison that kills the cat. The Copenhagen interpretation of quantum mechanics implies that after a while, the cat is simultaneously alive and dead. Yet, when one looks in the box, one sees the cat either alive or dead, not both alive and dead. This poses the question of when exactly quantum superposition ends and reality collapses into one possibility or the other.

Basically, there's a 50% chance that the poison is released which also means that the cat has a 50% chance to be alive and 50% chance to be dead. So, since the cat is in the box and you don't know whether or not the poison was released, the cat exists in 2 realities. It's both dead and alive. You don't know which is the true reality until you open the box and see the cat either dead or alive.